Radio communication systems, radar systems and image recognition systems are well known. Examples of such radio communication systems are broadband radio communication systems that transmit high data rates to and from customer premises, or that are utilized to connect a transmitter to a single or to multiple receivers. Examples of such radar systems include those used in military or civilian applications for the tracking of distant objects, such as aircraft or vessels, or those used within automobiles for tracking other vehicles on a highway to ensure that a safe distance is maintained at all times. Examples of such imaging systems include those used for screening vehicles or persons as an integral part of Homeland security initiatives.
The majority of the above examples operate with frequencies that are in the range of from 10 to 100 or 200 GHz, and utilize MMIC's comprising multiple circuit elements constructed on a single substrate. In many cases the MMIC is accompanied by additional discrete circuit elements that improve the performance of the overall system. Examples of such systems include multiple stage amplifiers used to amplify low level signals, for example a passive imager. Other examples of high frequency packaged systems include combined transmit/receive subsystems within a single cavity package, used for example in a radar system where a one needs to transmit and receive signals at similar frequencies.
As a result of the limited signal levels and the operating wavelengths associated with the above systems, there is a requirement to provide significant gains within the associated receiving system. These high gains are traditionally achieved using cascaded low noise amplifiers and filter chains which result in the associated circuit elements being sensitive to interference from parasitic noise that may be present within (or external to) the package. In addition, to operate at the high frequencies required, it is necessary to maintain relatively small distances between critical components integrated onto a single (or a limited number) of MMIC's contained in a single package. Such high gains and small distances place constraints on the design and the method of manufacturing the receiving elements of the above systems. Unwanted effects due to spurious radio frequency energy may couple into the MMIC's and be subject to positive feedback where energy is coupled from one section of the MMIC to another causing either a degradation in performance, or at the extreme, total failure of the system. Thus care must be taken both when designing the MMIC's and when placing them within the high frequency package to ensure that any spurious radio frequency energy present within the package is kept to a minimum.
European patent application EP1719175 (Powell) discloses the possible existence of resonant spurious energy in the form of standing waves present within a cavity package. This application discloses that the mode and resonant frequency of the spurious wave are determined by the dimensions of the package which encloses the circuit. Specifically one can obtain multiple standing waves within a given package depending on the dimensions. In cases where the resonant frequency associated with the standing waves lies within the operating bandwidth of the MMIC's contained within the package, there is a possibility of radiated energy coupling into sensitive parts of the circuitry causing degradation in its performance. One can also experience energy coupling from one part of a circuit to another via such standing waves. There also exists a danger of oscillation occurring in the case of coupling from the output of an amplifier to the input of the amplifier, or the input of other earlier stages of a number of cascaded amplifiers, or into critical passive elements associated with the amplifier.
Traditional methods of overcoming the problem of spurious oscillations include, for example, reducing the gain of each stage of the MMIC amplifier chain. Such a remedy has the unwanted effect of reducing the overall performance of the circuit. An alternative method of reducing the coupling spurious radio frequency energy is to introduce damping material such as foam into the cavity formed by the package. Another method of reducing the coupling of energy between, for example, a transmitter section to a receiver section of a complex MMIC is to use a low resistance shield which is connected via a low impedance path to the system ground.
Several methods and apparatus for reducing the feedback due to coupling of unwanted energy that may take place within the cavity of the package have been described in the art. For example international patent application number WO92/11665 titled ‘Three-dimensional microwave circuit carrier and integrated waveguide coupler’, Leicht et al. describes a method for shielding one MMIC from another using conductive screen(s) molded into the plastic lid of a non-hermetic package. The conductive screens are connected to earth via receiving posts on the MMIC substrate and may be attached with a conductive epoxy. The disadvantages of such a method are that it is limited to applications where there are multiple MMIC's comprising the gain stages of a receiver and the lid cannot be removed without damaging the MMIC. More importantly, such assemblies must be specifically designed for one/each application. This method also increases the spacing between the amplifiers comprising the system, thus limiting the maximum frequency at which the system can operate. The operation requires good ground contact being made between the underlying substrate and the conductive screens. In practice such a good ground contact may not be readily achieved due to manufacturing tolerances and/or parasitic inductance associated with the receiving posts on the MMIC substrate.
Another method is described by Uematsu Hiroshi et al in EP0798782 titled ‘Packaging for Microwave Circuits’. This application relates to the use of a dielectric spacer mounted directly on the module carrier on which the MMIC circuits are positioned. The dielectric spacer is intended to prevent the occurrence of coupling of unwanted RF between one MMIC and another. However it would be apparent to one skilled in the art that the dielectric spacers do not limit the occurrence of spurious wave mode within a cavity package. The use of dielectric within a cavity package also has the effect of making the package electrically larger, thus lowering the resonant frequencies associated with the package which may as a result be modified to an extent that they lie within the operating bandwidth of the active circuit. Another disadvantage of such a method is that it is limited to applications where there are multiple MMIC's comprising the gain stages of a receiver. The use of spacers also increases the distance between the MMIC, thus limiting the maximum frequency of operation.
Within the U.S. Pat. No. 5,416,668, Benzoni describes a novel method of constructing a well grounded shield between two distinct circuit elements comprising a high frequency transmitting/receiving system. The system utilizes a conductive shield, constructed as part of the lid of the package. The shield is earthed to the side walls of the cavity of the package utilizing integral mounting posts when the two section pieces of the package are assembled together. Thus the shield effectively prevents leakage of radio frequency energy from one sub cavity to another. However, the method fails to prevent or reduce the formation of parasitic standing waves within sub cavities formed by the shield, for example as described by Powell. The system also only operates if one can electrically isolate sub-elements of a complex system from each other without the shield interfering with the desired performance. Thus one can conclude that the ideas presented by Benzoni are limited in their range of applications, do not reduce energy associated with parasitic standing waves, and the shielding member(s) (and associated package) need to be custom designed for a specific system.
U.S. Pat. No. 5,608,188 (Multi Compartment Electromagnetic Energy Shield) in the name of Choon et al. discloses a method of constructing a well grounded shield between two distinct circuit elements comprising a high frequency transmitting/receiving system. The system utilizes a floating shielded member to ensure that the base of the shield is in close contact with an earth stripe that is placed between the two distinct parts system housed within the package. Thus, in a similar manner to that described by Benzoni, the shield effectively prevents leakage of radio frequency energy from one sub-cavity to another. To operate, the shield described by Benzoni is also dependent on the ability of electrically isolate sub element of a complex system one from the other without the shield interfering with the desired performance. Thus one can conclude that the ideas presented by Choon et al. are limited in their range of application, do not reduce energy associated with parasitic standing waves, and the shielding member(s) and associated MMIC again need to be custom designed for a specific system.
Within U.S. Pat. No. 6,862,001 titled ‘High Frequency Communication Device’ Kondoh et al. describe an alternative method for reducing the coupling of radio frequency energy from one circuit element to another utilizing a filter. Within the patent Kondoh et al describe the regular structure that reduces radio energy at the operating frequency of the enclosed MMIC and not spurious energy that is associated with the package itself. The frequency at which the filter operates is defined by the mark space ratio of the periodic mechanical structure that is built into the lid of package, and is specifically designed to coincide with the frequency of operation of the underlying circuitry. However, the structure can modify the desired operation of the underlying circuitry if the lower extremity of the periodic structure is in close proximity to the surface of the active circuitry. From the description presented it is clear that the filter is intended to reduce coupling of energy from one part of a circuit to another at the operating frequency of the underlying circuitry. Thus the periodic structure that forms the filter has to be specifically designed in conjunction with the MMIC.
US patent application number US20050274932 titled ‘Shielding for Electromagnetic Interference’ by Knight et al., presents an alternative method of reducing the energy coupling from one circuit element to another within a multiple MMIC system. The concept focuses on the use of a composite liquid crystal polymer loaded with an electrically conductive material to form a shield that is thermally matched to a low cost plastic cavity package. Knight et al. describe the action of designing the shielding member such that the lower extremity lies within a fraction of a wavelength of the underlying substrate onto which the active circuits are mounted. Maintaining such a minimum distance is intended to limit the leakage of radio frequency energy from one sub cavity to another at the operating frequency of the enclosed circuitry. Knight et al. further describe the anisotropic nature of the conductive material forming the shield which reduces the potential effect of the shield on the desired operation of the circuit elements. Although the structure described will reduce the coupling of energy (at the operating frequency) from one sub cavity to another, the shield and the underlying circuits have to be specifically designed in conjunction with each other to ensure that the shield does not impact the desired operation of the system.
As mentioned previously, Powell et al. (EP1719175), discloses the existence of a ‘spurious wave mode’ within the cavity of a high frequency package. The spurious wave is attributed to the resonant frequency of cavity dimensions and the associated materials. Powell et al. further describe a method and apparatus for limiting the amount of energy present in such spurious modes utilizing a resistive coated structure tuned to the impedance of the wave that one would experience if the structure were not present.
The use of the resistively coated structure reduces the effect of the interaction of such ‘spurious wave mode’ with sensitive parts of the enclosed circuitry. Powell et al. define that a number of different modes of different frequencies can be experienced within a cavity package, all of which are dependent on the dimensions of the cavity and describe the use of one or more partially conducting vane(s) protruding into the package cavity to reduce the energy associated with such waves. The number, size, orientation(s) of the resistive vanes are arranged to coincide with the points of maximum energy of the standing waves that would be present within the cavity if the vanes were not present. The resistance of the surface of the vanes is designed to match the impedance of the ‘spurious wave mode’ if the vanes were not present. Although the number, size, orientation and exact position of the vane(s) are stated to be of importance with regard to the extent to which the energy associated with a spurious mode wave is reduced, there are limitations as to the placement of the vanes to ensure that they do not interfere with the enclosed circuitry. In particular it is important to ensure that the dielectric structure onto which the resistive material is supported does not interfere with the operation of the system by loading sensitive areas of the underlying circuit. As with the disclosures mentioned above it is essential to design the resistive coated vanes in conjunction with the design of the enclosed MMIC.
Thus a key disadvantage of the methods and apparatus described by Powell is the accuracy required for placement of the vane(s), specifically to reduce higher order spurious wave modes. In addition, the dielectric structure supporting the resistive vane can either cause a loading effect on the circuitry or can themselves create new spurious wave modes which degrade the performance of a system that is operating at very high frequencies (i.e. >80-100 GHz).
Embodiments of the present invention seek to address one or more of the limitations of known packaging techniques and the systems outlined above and to preferably improve the performance thereof. More particularly, it is the aim of the present invention to provide a universal package for encapsulating high frequency electrical circuitry operating at frequencies above 10 GHz and which suppresses spurious wave modes within said package.
By use of the term ‘universal’ is meant that the package may be used with any high frequency electrical circuit (HFEC) and will function to suppress spurious wave modes regardless of the circuit that it used. The package or more specifically the damping structures therein act to suppress spurious wave modes in connection with any HFEC and such damping structures are not designed around a particular circuit. Thus, in contrast to the prior art packages, specific adaptations to, or prior knowledge of, the circuit design is not required for the damping structures to fulfill their role/function.